Toggle Grease Pencil "Use Lights" value using Python
You can use:
bpy.data.grease_pencils['GPencil'].layers['GP_Layer'].use_lights = False

The grease pencil object is:
gp = bpy.data.grease_pencils['GPencil']

Its 'GP_Layer' layer:
gp_layer = gp.layers['GP_Layer']

And its use lights property:
gp_layer.use_lights